1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a semiconductor device using a sapphire substrate used as an insulated board, wherein the sapphire substrate per se serves as device-to-device isolation regions, and a method of manufacturing the semiconductor device.
2. Description of the Related Art
Attention has heretofore been focused on a semiconductor device having an SOI (Silicon On Insulator) structure excellent in high-speed operability. Although several SOI structures have been put to practical use, a semiconductor device using a sapphire substrate is also one of them.
In a method of manufacturing the semiconductor device using the sapphire substrate, a silicon (Si) layer is first epitaxially grown on the sapphire (Al2O3) substrate. Next, Si+ ions are implanted therein to amorphize the silicon layer once, which in turn is monocrystallized again to thereby form a good-quality Si layer less reduced in crystal defect, whereby an SOS (Silicon On Sapphire) substrate ca be formed. Thereafter, a semiconductor device element is formed on the Si layer by using a normal semiconductor process.
A patent document (Japanese Unexamined Patent Publication No. Hei 5(1993)-326692) discloses a method of etching active regions of a silicon substrate, implanting oxygen ions to form an embedded oxide film and grinding the surface of the silicon substrate to thereby obtain silicon layers corresponding to the active regions, each having a uniform thickness.
A method has been described in a patent document (Japanese Unexamined Patent Publication No. 2000-21790), which in order to uniformly form a silicon layer on an insulated board at a low temperature, seeds a sapphire film to thereby precipitate silicon out of a low melting-point metal layer obtained by melting polysilicon or amorphous silicon. A method of seeding a sapphire film to thereby deposit silicon by a catalytic CVD method has been disclosed in a patent document (Japanese Unexamined Patent Publication No. 2000-223419).
When a semiconductor elemental device having an SOI structure is manufactured, a LOCOS (Local Oxidation Of Silicon) method is generally used in most cases as a method of isolating between respective devices or elements. A schematic cross-section of a conventional semiconductor device using a LOCOS method is shown in FIG. 4. Silicon layers 2, which serve as device regions, are formed on a sapphire substrate 1. A gate oxide film 4 of a field effect transistor, a gate 5 thereof and contacts 6 for its source and drain are formed over the silicon layer 2. Then, the elements (silicon layers 2) adjacent to one another are respectively separated by LOCOS oxide films 3.